1. Field of the Invention
The present invention relates to a defect inspection apparatus for detecting a defect of a measurement object such as an optical film and also relates to a defect inspection method adopted by the defect inspection apparatus.
2. Description of the Related Art
In recent years, a liquid-crystal display apparatus has been becoming very popular as a display apparatus representing television receiver sets and PCs (Personal Computers). The liquid-crystal display apparatus employs one of a variety of optical films to serve as the surface of a polarization plate. A main example of the optical films is a TAC (Tri-acetyl cellulose) film. In general, the optical film such as the TAC film is flexible and prone to injury. Thus, a hard coat layer 16 is typically created on the surface of a TAC film 15 as shown in FIG. 13, in order to improve the characteristic of being proof against strikes which may cause injuries. As shown in FIG. 14A, a hard coat layer 16 is laid on the surface of the TAC film 15 serving as the original film in a direction referred to as a moving direction. In order to lay a hard coat layer 16 on the surface of the TAC film 15, a coating section such as a blade 110 is used to apply a hard coat material continuously to the surface of the TAC film 15. When the hard coat material applied to the surface of the TAC film 15 hardens, a hard coat layer 16 is created on the surface of the TAC film 15.
While the hard coat material is being applied continuously to the surface of the TAC film 15, however, a portion of the coating section such as the blade 110 may be clogged up in some cases as shown by a mark X at a position P in FIG. 14B. Thus, coating unevenness is formed to have a line shape oriented in the moving direction of the blade 110 over the surface of the TAC film 15 as shown in FIG. 14B. As a result, after the hard coat material applied to the surface of the TAC film 15 hardens, a cord having a fixed length and a line shape oriented in the moving direction of the blade 110 over the surface of the TAC film 15 may be left in some cases on the hard coat layer 16 laid on the surface of the TAC film 15. If such a TAC film 15 is used in a light polarization device, the display characteristic of a liquid-crystal display apparatus employing the light polarization device may be badly affected in some cases so that improvement is desired.
By the way, there has been proposed a system to serve as a system based on an existing technology for acquiring information on a microstructure or information on the structure of the surface of a physical body such as a panel, a substrate or a wafer. The proposed system makes use of a sharing interferometer for splitting the wave surface of light radiated to an object of measurement into two partial wave surfaces and letting the two partial wave surfaces interfere with each other. For more information on the proposed system, the reader is advised to refer to documents such as Japanese Patent Laid-Open No. 2006-516737 (hereinafter referred to as Patent Document 1).
In addition, there has also been proposed a method to be used as a method for detecting a defect of an optical film which serves as an object of measurement. In accordance with the proposed method, an image of the optical film is taken by making use of typically a CCD (Charge Coupled Devices) camera and a defect of the optical film is detected by detecting color-tone changes or shading changes. For more information on the proposed method, the reader is advised to refer to documents such as Japanese Patent Laid-Open No. 2006-208196 (hereinafter referred to as Patent Document 2).